Digital Electronics: Principles, Devices and Applications

332 Digital ElectronicsDQO/P4-inputLUTClockclkFFFigure 9.27Logic block of a typical FPGA.Logic blocks can have more than one LUT and flip-flops also to give them the capability ofrealizing more complex logic functions. Figure 9.28 shows the architecture of one such logic block.The architecture shown in Fig. 9.28 is that of a logic block of the XC4000 series of FPGAs fromXilinx. This logic block has two four-input LUTs fed with logic block inputs and a third LUT thatcan be used in conjunction with the two LUTs to offer a wide range of functions. These include twoseparate logic functions of four inputs each, a single logic function of up to nine inputs and manymore. The logic block contains two flip-flops.Figure 9.29 shows another similar LUT-based architecture that uses multiple LUTs and flip-flops.The architecture shown in Fig. 9.29 is that of a logic block called a programmable function unitC1 C2 C3 C4SelectorG4G3G2G1Look-upTable(LUT)S/RControlD DD QECRDQ2InputsLook-upTable(LUT)'1'GF4F3F2F1ClockLook-upTable(LUT)S/RControlDD QDECRDQ1'1'FFigure 9.28Logic block architecture of the XC4000 FPGA from Xilinx.